A typical rectilinear motion proportional solenoid produces an output force which is non-linearly proportional to the applied electrical current input. In order to achieve linear displacement output, the currently available solenoids use some combination of non-linear spring configurations with their attendant characteristics balanced against a generated force which is non-linear with regard to electrical current input, the combination of non-linear features resulting in linear characteristics. Factory assembly and calibration are required in order for these solenoids to achieve the desired results, and when properly assembled and calibrated, the resulting output is linearly proportional to a certain defined current input.
This arrangement is adequate where a constant gain or rate is needed, rate being defined as the output displacement versus current input; however, where it is necessary to vary the rate of the armature displacement to achieve differing outputs, and linearly proportional motion is desired, these solenoids must be disassembled and the internal components must be adjusted to compensate for the desired changes in rate of displacement. The required adjustments are entirely internal to the solenoids and consist either in changing the spring rate through cam adjustments, adjusting the spring tension through repositioning, or adding or removing shims to effect changes. These are all structural changes which require physically contacting and displacing the movable armature to effect a change in performance, and any adjustments are extremely critical when working with such small distances and close tolerances as are found in solenoids of this type. Furthermore, since the very act of adjusting the armature position displaces a movable entity which is the control point or output position set point of the solenoid, proper adjustment is difficult, if not impossible. Thus, after reassembly, the unit must be tested to assure that linear output has been retained. If it has not been retained, disassembly and repositioning are indicated along with further testing after reassembly to determine whether or not the desired output has been achieved.
The difficulties encountered in making adjustments are such that most solenoids of this design are not made to be adjustable. They are designed instead to provide a specific rate of output displacement, and if a different output is required, a different solenoid must be used. Where adjustability is indicated in a spring-supported system for obtaining linear motion in an electro-mechanical device, as in U.S. Pat. No. 3,727,900 to Casey, a non-linear spring is used, adjustment is made to the spring rate through the addition or removal of shims, disassembly is required, and the movable armature must be contacted and physically displaced, introducing uncertainty into the control point position by the very act of adjustment. Thus, adjustment of the solenoid while coupled to the driven component, be it a valve, transducer, or other device, either at rest or while operating, is not normally possible with these solenoids.
The force generated by these solenoids and applied across the magnetic core gap to the armature is a function of the changing magnetic circuit permeance and the flux density. The magnetic circuit permeance changes with changes in the core gap or the armature stroke even with constant magnetic induction forces, resulting in definite non-linear force characteristics. Since the non-linear springs of this design are balanced against non-linear forces, with a changing force/gap curve dependent upon the magnetic circuit permeance, attempted adjustments for gain will degrade the linearity characteristics.
An additional problem encountered by the type of proportional solenoid currently available is the changes induced in the gap lengths and in the armatures themselves by expansion or contraction of the support springs, the length of the housings, or the internal components due to temperature variations. These variations also degrade the linearity characteristics due to the resulting changes induced in the output position set point or control point of the solenoid armature.